Method and apparatus for convection brazing of aluminum heat exchangers

ABSTRACT

A convection braze furnace for brazing aluminum heat exchangers in an inert gas rich atmosphere includes entrance and exit vestibules forming atmosphere barriers of suspended stainless steel strips. The interior of the braze furnace is divided into multiple zones for progressively heating the heat exchangers to a brazing temperature and then cooling the heat exchangers in the final zone. An impeller circulates the heated intent gas atmosphere within each zone to accelerate heat transfer. A chain type conveyor supports the heat exchangers as they are moved through the braze furnace. An isolated return tube surrounds the lower return side of the conveyor chain as it passes through the braze furnace. The braze furnace housing is comprised of inner and outer shells having an inert gas pressurized cavity interstitial therebetween. The inner shell includes a plurality of expansion strips having generally ellipsoidal corner expansion joints.

TECHNICAL FIELD

The subject invention relates to a convection braze furnace for brazingaluminum heat exchangers in an oxygen free atmosphere.

BACKGROUND ART

Heat exchangers are used in various capacities in automotiveapplications. For example, all automobiles having water cooled enginesemploy a radiator and a heater core. Automobiles equipped with airconditioning also include an evaporator and a condenser. These heatexchangers are made from aluminum and consist of two spaced header tanksinterconnected by flow tubes having cooling fins extending therefrom.Fluid is circulated through the header tanks and flow tubes to effectthe necessary temperature drop.

The header tanks and flow tubes and cooling fins are rigidly attached toone another by brazing. It has been found that this brazing operationcan be most efficiently accomplished in an oven for mass productionapplications. The prior art teaches the use of radiant braze ovenswhereby the heat exchangers are fed through a heated muffle tube withradiant heat energy being used to raise the temperature of the heatexchangers to the braze liquification temperature.

The prior art has found that by using the principles of convection heattransfer, the heat exchanger workparts can be more efficiently andquickly raised to the braze liquification temperature. Examples of theseprior art convection braze furnaces may be found in U.S. Pat. No.3,756,489 to Chartet, issued Sep. 4, 1973, U.S. Pat. No. 3,882,596 toKendziora et al, issued May 13, 1975, and U.S. Pat. No. 4,501,387 toHoyer, issued Feb. 26, 1985. The Chartet and Kendziora et al referencesdisclose sectioning the braze furnace into zones, with the final zonecomprising a cooling zone for solidifying the braze material prior tothe workparts exiting the braze furnace. However, these prior art brazefurnaces are deficient in that the rate of heat energy absorbed from theworkparts in the final cooling zone could not be accurately controlled.If the workparts are cooled too quickly, warpage will occur. Conversely,if the workparts are not cooled quickly enough, the workparts will exitthe braze furnace with the braze material in a partially congealed statebonding to the atmosphere curtain in the exit vestibule. Hence, precisecontrol over the rate of workpart cooling in the prior art is notavailable.

SUMMARY OF THE INVENTION AND ADVANTAGES

A convection braze furnace assembly for brazing aluminum heat exchangerworkparts is provided. The assembly comprises a housing defining acontrolled atmosphere brazing chamber, a conveyor means for conveyingworkparts through the housing, an impeller for circulating atmospherethrough the workparts to establish an atmosphere convection currentwithin the housing, and a heater means disposed in the convectioncurrent for elevating the atmospheric temperature of the convectioncurrent. The improvement of the subject invention comprises anadjustable heat absorber means disposed in the convection current andspaced from the heater means for absorbing heat energy from theconvection current while the heater means elevates the atmospherictemperature of the convection current to precisely control theatmospheric temperature of the convection current thereby improvingregulation of braze liquification and solidification within the housing.

The subject invention overcomes the deficiencies in the prior art by theadjustable heat absorber means acting in concert with the heater means.Whereas the prior art included only a heat absorber means in the finalcooling zone to absorb heat from the convection current, the subjectinvention employs both the heater means and the heat absorber means intandem for more quickly responding to temperature variation in theconvection current and precisely absorbing the correct amount of heatenergy from the convection current to avoid warpage and distortion ofthe workpart while correctly reducing the temperature of the workpart toa proper braze solidified temperature. Hence, workpart solidification isobtained in time and braze furnace length than the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a simplified cross-sectional view of a convection brazefurnace according to the subject invention;

FIG. 2 is a cross-sectional view of the braze furnace as taken alonglines 2--2 of FIG. 1;

FIG. 3 is an exploded perspective view of the flow control louver andflow adjustor plate according to the subject invention;

FIG. 4 is an enlarged fragmentary view of an opening in the flow controllouver superimposed over an aperture in the flow adjustor plate;

FIG. 5 is a front view of the expansion strips and expansion jointsaccording to the subject invention;

FIG. 6 is a side view as taken along lines 6--6 of FIG. 5;

FIG. 7 is a perspective view of an expansion joint;

FIG. 8 is a fragmentary perspective view showing the expansion bellowssurrounding each of the conveyor return tubes;

FIG. 9 is a perspective view of the combination cooling tube and burnertube arrangement according to the subject invention; and

FIG. 10 is a perspective view of an alternative embodiment of the flowcontrol louver assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figures, wherein like numerals indicate like orcorresponding parts throughout the several views, a controlledatmosphere braze furnace assembly is generally shown at 10. The brazefurnace assembly 10 is particularly adapted for brazing aluminum heatexchanger workparts utilizing convection heat transfer. It is well knownthat forced convection heat transfer is a much more efficient method ofheating objects than either radiant heat transfer or natural convectionheat transfer. Accordingly, the subject braze furnace 10 utilizes forcedconvection technology to accelerate brazing of aluminum heat exchangers,such as radiators, condensers, heater cores, and evaporators forautomotive applications.

The subject furnace assembly 10 includes a substantially sealed housing,generally indicated at 12 in FIGS. 1 and 2, defining therein acontrolled atmosphere brazing chamber 14. Preferably, the controlledatmosphere comprises nitrogen because it is inexpensive, nontoxic, andinert to the components associated with brazing aluminum heatexchangers. However, other types of inert gases are applicable so longas the brazing chamber 14 remains nearly completely purged of oxygen.Preferably, the oxygen content within the brazing chamber 14 ismaintained at approximately 2 parts per million.

The brazing chamber 14 within the housing 12 is divided into a pluralityof successive interior braze zones 16, 18, 20, 22, 24, and 26 generallyisolated from one another. However, any number of zones between two andeight may be used. The zones proceed in succession aligned from aforward end 28 of the housing 12 adjacent zone 16 to a rearward end 30adjacent zone 26. Extending longitudinally from the forward end 28 ofthe housing 12 is an insulated entrance vestibule 32 including aplurality of thin, narrow stainless steel strips 34 forming anatmosphere barrier curtain. Likewise, an exit vestibule 36 extendslongitudinally from the rearward end 30 of the housing 12 and includes aplurality of suspended strips 38 forming another atmosphere barriercurtain. As will be described in greater detail subsequently, a burnertube is disposed in a convection current within each zone 16-26, and aninjection tube 39 in each zone is disposed on an upstream side of eachburner tube and a downstream side of the workparts W so that en emergingstream of room temperature nitrogen from each injection tube 39 isheated over the burner tube almost immediately upon entering the brazingchamber 14, and well before circulating to the workparts W. Theinjection tubes 39 have multiple openings therein for emitting nitrogeninto the brazing chamber 14, as shown in FIG. 2.

The housing 12 comprises an inner shell, generally indicated at 40,sealed substantially gas tight and defining a boundary for the brazingchamber 14. Hence, the inner shell 40 contains without leakage the inertcontrolled atmosphere within the brazing chamber 14, except, of course,sacrificial leakage from the entrance 32 and exit 36 vestibules. Anouter shell 42 sealably surrounds the inner shell 40 and defines aninterstitial space 44 between the outer shell 42 and the inner shell 40.As illustrated in FIGS. 1 and 2, the interstitial space 44 issubstantially entirely filled with an insulated material 46, such asrock wool.

An intersticial pressurizer means, generally indicated at 48, isprovided for pressurizing the intersticial space 44 with an inert gas ata pressure above the pressure within the brazing chamber 14 such that agas leak accidentally occurring in the inner shell 40 causes a flow ofthe inert gas from the intersticial space 44 into the brazing chamber 14to prevent contamination of the controlled atmosphere of predominatelyinert gas within the inner shell 40. Preferably, the intersticialpressurizer means 48 provides the same inert gas, i.e., nitrogen, to theintersticial space 44 as the controlled atmosphere within the brazingchamber 14. Therefore, if a crack or tear in the inner shell 44 occursdue to thermal expansion and contraction, fatigue, etc., only inert gaswill escape into the brazing chamber 14. In this manner, the controlledatmosphere of oxygen free inert gas is not compromised.

The intersticial pressurizer means 48 includes a supply conduit 50extending through the outer shell 42 and communicating with theintersticial space 44, as shown in FIGS. 1 and 2. An inert gas source 52is operatively coupled to the supply conduit 50 for supplying the inertgas under pressure to the intersticial space 44. Spaced from the supplyconduit 50, preferably on the opposite end of the housing 12, isprovided a discharge conduit 54 operatively associated with a suctionpump 56. An inert gas receptacle 58 receives inert gas from the suctionpump 56. Alternatively, an exhaust hood (not shown) associated with eachof the entrance 32 and exit 36 vestibules can be utilized to create thenecessary suction of the suction pump 56, in which case the dischargeconduit 54 would tap directly into one or the other of the exhausthoods. The suction pump 56 is provided to assist in surrounding theentire inner shell 40 with inert gas. Because of the tortious path inthe intersticial space 44 created by the insulation 46, the suction pump56 is effective to draw the inert gas gently and completely through theintersticial space 44. In this manner, the entire braze chamber 14 issurrounded by a plenum of nitrogen and, in the event of a breach in theinner shell 40, only nitrogen will seep into the oxygen free brazechamber 14. As will described in detail subsequently, a plurality ofimpellers 76 are disposed within the braze chamber 14 for circulatingthe nitrogen atmosphere. The suction side of each impeller 76 createpressure drop which is useful in drawing nitrogen into the brazingchamber through any cracks in the inner shell 40 which occur in areaswhere the atmospheric pressure in the intersticial space 44 is greaterthan in the braze chamber 14. However, any cracks in the inner shell 40which occur in areas where the atmospheric pressure in the interstitialspace 44 is less than in the braze chamber 14, e.g., near the dischargeside of the impellers 76, will result in nitrogen from the braze chamber14 moving into the interstitial space 44.

A heater means 60 is disposed in each of the zones 16-26 for elevatingthe atmospheric temperature within each of the respective zones 16-26.Preferably, the heater means 16 comprises a flat serpentine style burnertube supplied with a heated fluid flow from an exterior burner 62. Theburner tubes may include 6 passes, on four passes as shown. However,additional or fewer passes may be utilized depending upon the particularor anticipated workpiece mass heat load. The burner tubes are supportedhorizontally approximately seventeen inches, on center, above the floorof the inner shell 40.

A conveyor means, generally indicated at 64 in FIGS. 1 and 2 provides aplanar workpart support surface along an upper straightaway 66 thereoffor successively conveying workparts W, shown in phantom in FIG. 1,through the housing 12 supported flat against the support surface of theupper straightaway 66. More particularly, the conveyor means 64comprises four endless flexible conveyor elements arranged in paralleland movably disposed within the housing 12. However, additional or fewerconveyor elements may be used depending upon the size of the workparts Wso as to provide adequate support. The four conveyor elements extendoutwardly from the housing 12, beyond the forward 28 and rearward endthereof, for circulatory movement through the housing 12. Hence, eachconveyor element is moveable through the respective upper straightaway66 where it supports and conveys heat exchanger workparts W through thehousing 12, and also moveable through a lower return straightaway 68.Preferably, the four conveyor elements of the conveyor means 64 eachcomprise conventional roller chains which facilitate synchronizeddriving by four corresponding drive sprockets 70. The drive sprockets 70are supported on a common axel and rotated by a motor 72 for maintaininga tension along the respective upper straightaways 66 as the workparts Ware moved therealong. An idler sprocket 74 is disposed outwardly fromthe entrance vestibule 32 for maintaining tension and proper orientationalong each of the conveyor elements.

An impeller 76 is disposed within each of the zones 16-26 forconvectively circulating atmosphere through the workparts W supported onthe upper straightaway 66 of the conveyor means 64. Hence, the impellers76 each create an atmospheric convection current within the respectivezones 16-26 in the housing 12. As shown in FIGS. 1 and 2, the impellers76 are each supported on a rotating shaft 78 extending through the roofof the inner shelf 40, intersticial space 44, and outer shell 42. Eachshaft 78 is rotatably driven by a motor 80 disposed within a watercooled jacket (not shown). The impeller 76, shaft 78, and motor 80assembly of each zone 16-26 is sealed with the housing 12 to preventoxygen infiltration to either the intersticial space 44 or the brazingchamber 14. The impeller 76 is of a radial discharge type and generallyresembles a paddle wheel having a plurality of radially extending fins.Hence, as shown in FIG. 2 by the convection current arrows, atmosphereis discharged radially and horizontally from the impeller 76 anddirected in a circulating path through the brazing chamber 14 and backto the impeller 76.

An atmosphere director means, generally indicated at 82 in FIG. 2, isdisposed within the housing 12 for directing the circulating atmosphereperpendicularly toward the support surface (i.e., upper straightaway 66)of the conveyor means 64 so that an atmosphere momentum created by thecirculating atmosphere urges the workparts W tightly against theconveyor support surface to prevent lateral and torsional movement ofthe workparts W on the support surface as the impeller 76 circulates theatmosphere at highly elevated velocities. The atmosphere director means82 includes that portion of duct work structure within the brazingchamber 14, disposed between a pressure outlet 84 of the impeller 76 andthe support surface (i.e., upper straightaway 66) of the conveyor means64 which causes the flow of circulating atmosphere to impinge straightdownwardly, or normally, toward the upper straightaway 66. In thismanner, there are no vector components of momentum flow urging theworkparts W to move either laterally or torsionally relative to theconveyor means 64. Without the atmosphere director means 82, thecirculating atmosphere would engage the workparts W along directionsskewed to the upper straightaway 66 of the conveyor means 64 whereby themomentum of the circulating fluid would cause the workparts W to jiggleor shift upon the conveyor means 64 with the resultant brazed heatexchanger having imperfectly aligned components. That is, the workpartsW are laid-out on the conveyor means 64 with each component, e.g.,header, fins, tubes, etc., aligned relative to the next in an exactlydetermined relationship. If the workparts W are jostled prior to thebraze material liquefying and then resolidifying, then the resultingfinal heat exchanger will be deformed. For this reason, man prior artbraze furnaces require the workpart to be held motionless in a jigfixture while travelling through the furnace. Or, if no jig fixture isused, only permit very low convection current velocities.

The atmosphere director means 82 includes a flow control louver 86 asbest shown in FIG. 3. The flow control louver 86 has a plurality ofdecrete openings 88 therein. The flow control louver 86 is disposedwithin the housing 12 between the pressure outlet 84 of the impeller 76and the support surface of the conveyor means 64. The flow controllouver 86 effectively straightens the flow of circulating atmosphereemanating from the impeller 76 so that in the area of the workparts Wthe atmosphere flows perpendicularly to the upper straightaway 66 of theconveyor means 64.

An adjustor means, generally indicated at 90 in FIGS. 3 and 4 isprovided for adjusting the respective areas of each of the discreteopenings 88 in the flow control louver 86 to more accurately control thevelocity of circulating atmosphere within the brazing chamber 14. Moreparticularly, the adjustor means 90 includes a flow adjustor plate 92slideably disposed adjacent the flow control louver 86 and having aplurality of apertures 94 therein corresponding in size to the discreteopenings 88 in the flow control louver 86. A plurality of axial bolts 96interconnect the flow adjustor plate 92 and the flow control louver 86for permitting sliding movement of the flow adjustor plate 92 relativeto the flow control louver 86. The apertures 94 in the flow adjustorplate 92 are superimposed over the discrete openings 88 in the flowcontrol louver 86 so that relative movement between the adjustor plate92 and the flow control louver 96 creates intersections between thesuperimposed apertures 94 and discrete openings 88, which intersectionsestablish the exclusive path of convection current movement through thatportion of the brazing chamber 14. In FIG. 2, the flow control louver 86and flow adjustor plate 92 are shown disposed upon spaced supportbrackets supported horizontally within the brazing chamber 14.

An alternative embodiment of the adjustor means 90' is illustrated inFIG. 10. As shown, the adjustor means 90' comprises a plurality ofindividual strips 922' each separately adjustable with respect to a slotin the flow control louver 86'. This alternative adjustor means 90'permits a control of the atmosphere flow within a zone so that a higherpressure is created adjacent the exit side of each zone to causeatmosphere therein to always flow toward the entrance vestibule 32.

The atmosphere director means 82 further includes a duct work partition98 having a generally vertically extending orientation within thebrazing chamber 14. The duct work partition 98 extends downwardly fromthe pressure outlet 84 of the impeller 76 to an angled inner accesspanel 100 disposed substantially below the impeller 76. Flowing highpressure atmosphere exiting the impeller 76 is directed through thepressure outlet 84 and contained between the inner shell 40 and theinner access panel 100 prior to entering the flow control louver 86. Thetemperature of the convection current in this area is measured by athermo couple-type sensor 101 extending through the outer shell 42, theinterstitial space 44, and the inner shell 40. The flow control louver86 straightens the flow of the convection current at that point todirect the current perpendicularly toward the upper straightaway 86 ofthe conveyor means 64 so that the workparts W are held tightly againstthe conveyor means 64 without shifting or movement. After passingthrough the workparts W, the convection current continues in a downwardmotion through the remainder of the conveyor means 64 comprising thereturn straightaway 68 and downwardly across one end of the heater means60. The convection current then contacts the floor of the inner shell 40and is directed back upwardly, once again across a distal end of aheater means 60 and thence between the duct work partition 98 and anouter access door 102 to a suction inlet 104 of the impeller 76. Hence,as viewed from the cross section of FIG. 2, the atmosphere is circulatedin a clockwise motion within the brazing chamber 14. The outer accessdoor 102 is secured to the outer shell 42 by wing nuts 106 forpermitting entrance into the brazing chamber 14 for repair andmaintenance purposes.

Referring now to FIGS. 2 and 5-7, the inner shell 40 is shown comprisinga plurality of tubular shell sections 108 arranged in end-to-endfashion. As particularly shown in FIG. 7, each of these shell sections108 have a pair of straight side portions 110 merging at alongitudinally extending corner 112 therebetween. In the preferredembodiment, each of the tubular shell sections 108 have a quadrilateralcross section defined by four transverse side portions 110 separated byfour interposed corners 112. Hence, the inner shell 140 formed by theend-to-end arrangement of the shell sections 108 has a generallyelongated rectangular box shape. The shell sections 108 each relate toand define the respective zones 16-26. A vertically extending partition114 isolates one zone from the next adjacent zone. Each partition 114has an opening therein (not shown) just large enough for the workparts Wto pass through. The ductwork partitions 98 support the partitions 114in the proper orientation.

Because the temperature in each zone 16-26 will generally vary from zoneto zone, the thermal expansion and contraction of the various shellsections 108 will differ. For this reason, four spaced apart sheet-liketransversely extending expansion strips 116 are disposed betweenadjacent pairs of the shell sections 108 for flexibly absorbing thermalexpansion and contraction of the shell sections 108 withoutcontaminating, or compromising, the controlled oxygen free atmosphere inthe housing 12. The expansion strips 116 are respectively attached toopposing side portions 110 of adjacent shell sections 108 in order tointegrate the shell sections 108 together to form the inner shell 40.

As best shown in FIG. 7, the expansion strips 116 have a continuouscross-section along their length, such as created by roll forming.Particularly, each expansion strip 116 includes a U-shaped centralportion 118 forming a flexible spring-like, or pleat-like, orbellows-like, member adjoining the adjacent shell sections 108. A pairof flank portions 120 extend laterally from each side of the U-shapedportion 118 and provide overlapping contact with the side portions 110of the opposing shell sections 118. The flank portions 120 are welded tothe respective side portions 110 to unify the structures and provide anair-tight seal therebetween. A rib 122 extends perpendicularly upwardlyfrom each of the flank portions 120 to rigidify the structure.

Four expansion joints 124 are respectively disposed between adjacentpairs of the expansion strips 116 and opposing pairs of corners 112 ofthe shell sections 108 for simultaneously absorbing thermal expansionand contraction from the expansion strips 116 and expansion andcontraction from the shell sections 108 without contaminating orcompromising the controlled atmosphere in the housing 12. Moreparticularly, as best shown in FIG. 7, each expansion joint 124 has afragmentary generally ellipsoidal shape somewhat resembling a partialhollow disk or, more colloquially, an inflated pita-style bread loaf.The expansion joints 124 are generally sector-shaped having an innercavity which encases the U-shaped portions 118 of each of the adjoiningexpansion strips 116. The expansion joints 124 further include agenerally circular outer curvature centered on an axis which iscoincidental with, or at least parallel to, the corners 112 of the shellsections 108. This outer curvature has a radius larger than the width ofthe U-shaped central portion 118 of the expansion strips 116. As shownin FIG. 7, the expansion strips 116 are each cut back from the corners112 of the shell sections 108 to establish a gap which is bridged by theexpansion joints 124. The expansion joints 124 are welded air tightobliquely across each of the U-shaped portions 118 of the respectiveexpansion strips 116 and to the edges of the opposing corners 112 of theshell sections 108. In this manner, the integrity of the inner shell 40is maintained so as to prevent contamination of the controlledatmosphere with oxygen.

Preferably, the expansion strips 116 and expansion joints 124 arefabricated from the same or similar material as the shell sections 108.A light gauge stainless steel has been found to provide satisfactoryresults. Each expansion joint 124 is formed in two separate parts weldedtogether along a peripheral seam, as exemplified in FIG. 6.

The expansion joints 124 are particularly useful in permitting expansionand contraction of the expansion strips 116 in a transverse direction,i.e., transverse to the movement of the conveyor means 64, whilesimultaneously providing for longitudinal expansion and contraction ofthe shell section 108. The large outer curvature of the expansion joints124 is effective in reducing the stress concentrations encounteredduring thermal expansion and contraction of the various components.Hence, the expansion strips 116 and expansion joints 124 operate inconjunction with the shell sections 108 to maintain a substantially gastight sealed inner shell 40 during the heat-up and cool-down phases ofoperation.

Referring now to FIGS. 1 and 8, the conveyor means 64 is shown includingthe return straightaway 68 disposed below and parallel to the upperstraightaway 66, both of which are disposed within the heated atmosphereof the brazing chamber 14. Because the ends of the conveyor means 64extend outwardly beyond the forward 28 and rearward 30 ends of thehousing 12, the flexibile conveyor elements are susceptible to capturingand drawing oxygen into the controlled atmosphere of the brazing chamber14. To substantially reduce this undesirable occurrence, the subjectbraze furnace assembly 10 includes a tubular guide means, generallyindicated at 126, which extends gas tight through the housing 12 and isoperatively gas tight sealed adjacent the forward 28 and rearward 30ends of the housing 12 for guiding the lower return straightaway 68 ofthe conveyor means 64 through the housing 12 while preventing ambientatmosphere entrance to the housing 12 to maintain heat energy of theconveyor means 64 moving through the lower return straightaway 68without contaminating the controlled atmosphere in the brazing chamber14.

The tubular guide means 126 includes a return tube 128 having agenerally square cross-section extending completely through the housing12 and operatively surrounding the lower return straightaway 68 of theconveyor means 64. That is, the return tube 128 extends through theforward end 28 of the housing 12, including the outer 42 and inner 40shells, through each of the zones 16-26, and again through the inner 40and outer 42 shells adjacent the rearward end 30 of the housing 12. Thereturn tube 128 includes a first end 130 rigidly gas tight sealed, e.g.,by welding, to the forward end 28 of the housing 12. Because of thermalexpansion and contraction, a thermal expansion means comprising anexpansion member 132 surrounds a second end 134 of the return tube 128and is operatively gas tight sealably coupled between the return tube128 and the exterior outer shell 42 of the housing 12 for permittingrelative movement between the return tube 128 and the housing 12. Thatis, due to thermal expansion and contraction differences between thehousing 12 and the return tube 128, the return tube 128 can not befixedly attached at both its first 130 and second 130 ends to therespective forward 28 and rearward 30 ends of the housing 12. Therefore,the expansion member 132 is provided for permitting relative movementbetween the return tube 128 and the housing 12 due to thermal expansionwhile preventing ambient air entrance into the housing 12. The expansionmember 132 comprises a bellows having a series of alternatingfrustoconical sections collapsible upon itself in typical accordionstyle. In this manner, the length of the return tubes 128 can extend orcontract irrespective of the movement of the housing 12. Also, thereturn tubes 128 absorb heat energy due to their disposition within theconvection current and radiate that heat energy to the conveyor elementstherein to help maintain the conveyor elements at an elevatedtemperature.

To eliminate air or atmosphere movement between the intersticial space44 and the brazing chamber 14, a tubular sleeve 136 surrounds each ofthe return tubes 128 within the intersticial space 44 between the inner40 and outer 42 shells, as shown in FIGS. 2 and 8. The sleeves 136 areeach welded at their respective ends to maintain a fluid tight gas seal.Hence, the sleeves 136 provide a protective jacket, or cover, about thereturn tubes 128 within the intersticial space 44.

In the preferred embodiment shown in FIG. 1, the first five zones 16-24comprise successive heating zones for raising the temperature of theworkparts to a point where the flux melts and then the braze materialmelts and flows into the necessary crevices to effectuate completebrazing. However, the final zone 26 comprises a cooling zone 26 whereinthe temperature of the workpart is brought below the solidificationtemperature of the braze material. The braze material must betransformed from the liquid phase to the solid phase prior to enteringthe exit vestibule to prevent bonding between the curtain strips 38 andthe braze material. Yet, the temperature in the cooling zone 26 must notdrop too rapidly or else the workpart W will warp. Hence, cooling of theworkparts to the solidification temperature of the braze material mustbe accomplished in a precisely controlled manner. In order to carry thisout, the cooling zone 26 is provided with both a heater means 60 andalso an adjustable heat absorber means 138 for absorbing heat energyfrom the convection current while the heater means 60 elevates theatmosphere temperature of the convection current to precisely controlthe atmospheric temperature of the convection current thereby improvingregulation of braze liquification and solidification within the housing12. More particularly, as best shown in FIG. 9, the heat absorber means138 includes a cooling tube arranged in serpentine fashion and spaceddirectly above the burner tube of the heater means 60. Preferably, thecooling tube and burner tube are staggered slightly from one another sothat the convection current is adequately circulated therebetween. Acooling unit 140 is operatively associated with the cooling tube andspaced outside the housing 12. In FIG. 1, a simple manual adjustmentdevice 142 is illustrated to exemplify the adjustability of the heatabsorber means 138. However, each of the burners for the various heatermeans 60 are likewise adjustable.

In order to precisely and accurately control the atmospheric temperatureof the convection current to ensure solidification of the braze materialand to prevent warpage of the workpart, both the heater means 60 and theheat absorber means 138 are operated simultaneously in the cooling zone26. In a manner similar to the heater means 60 provided in the heatingzones 16-24, the heater means 60 and heat absorber means 138 within thecooling zone 26 are supported in a removable support plate 144 whichextends through the outer housing 42, the intersticial space 44, and arepresented substantially flush with the inner shell 40. The support plate44 is attached, such as by screws or the like, to the housing 12 forpermitting convenient removal of the heater means 60 and the heatabsorber means 138.

In operation, workparts W are placed prefluxed on the upper straightaway66 of the conveyor means 74 just in front of the entrance of thevestibule 32. The headers of the heat exchanger workparts W are alreadyheated to approximately 300° F. to 1000° F. when placed on the conveyormeans 64 due to the prior drying of the flux in a nearby drying oven.However, the cooling fins of the heat exchanger workparts W aresignificantly cooler then the headers. Once placed on the conveyor means64, the workparts W are moved into the entrance vestibule 32 and throughthe suspended strips 34 therein until emerging from the entrancevestibule 32 into the first zone 16. An oxygen purging flow of nitrogenbathes the workparts W while in the entrance vestibule 32 to preventoxygen contamination within the brazing chamber 14. Within the firstzone 16, the circulating convection current is forced downwardly throughthe workpart W at a very high velocity, on the order of 3500 feet perminute, to bring the temperature of the entire workpart W up toapproximately 1000° F. Within the next two heating zones 18, 20, theworkpart W is raised to a temperature in which the flux becomes moltenand flows into the crevices to be braze. As the workpart W continuesalong with the conveyor means 64, the temperature is gradually raised ineach zone until reaching the fourth and fifth heating zones 22, 24wherein the braze material becomes liquified and fully and completelyflows into the predetermined joints. From the fifth heating zone 24, theworkpart W is conveyed to the cooling zone 26 where a much lowertemperature convection current is circulated through the workpart W. Theheat absorber means 138 operates in concert with the heater means 60disposed in the cooling zone 26 to provide a precisely controlledtemperature in which to solidify the braze material. Hence, upon exitingthe cooling zone 26, the braze material of the workpart W is completelysolidified and enters the exit vestibule 36 without adhering to thesuspended strips 38 therein.

Because the braze furnace assembly 10 is structured to provide extremelyhigh velocity convection current, the workparts W can be quickly cycledthrough the braze furnace assembly 10. Hence, whereas the prior artbraze furnaces typically require on the order of twenty minutes or moreto cycle each workpart, the subject braze furnace assembly 10 is capableof cycling the same workpart W in approximately two to ten minutes.Also, due to the high velocity convection currents, the subject brazefurnace assembly 10 is capable of brazing a mixture of workpart W modelswhile still providing satisfactory braze results. That is, largeworkparts such as radiators can be cycled in the braze furnace assembly10 along with small heater cores and evaporators. This, again, is due tothe high velocity atmosphere circulation permitted by the atmospheredirector means 82. Hence, the subject braze furnace assembly 10 is asignificantly more efficient and versatile braze furnace assembly thanthose presently found in the prior art.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology which has been used is intended to bein the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A convection braze furnace assembly for brazingaluminum heat exchanger workparts, said assembly comprising:a housinghaving a plurality of successive interior heating zones generallyisolated from one another and preceding a terminal cooling zone;conveyor means for sequentially conveying workparts to each of saidheating zones and said cooling zone; an impeller for establishing anatmospheric convection current in each of said heating zones and saidcooling zone; a burner type disposed in each of said heating zones andsaid cooling zone for elevating the atmospheric temperature of therespective said convection current; and a cooling tube disposed in saidconvection current of said cooling zone and spaced from the respectivesaid burner tube for improving regulation of braze solidification withinsaid cooling zone; said cooling tube and said respective burner tube insaid cooling zone being mutually supported on said housing by a unitarysupport plate removably secured to said housing.